The present invention relates to overvoltage protection of electrically isolated systems. More particularly, the present invention relates to an improved electrostatic discharge (“ESD”) apparatus that may be employed in Ethernet systems.
Electrical overstress transients (“EOS transients”) produce high electric fields and high peak powers that can render circuits, or the highly sensitive electrical components in the circuits, temporarily or permanently non-functional. EOS transients can include transient voltages or current conditions capable of interrupting circuit operation or destroying the circuit outright. EOS transients may arise, for example, from an electromagnetic pulse, an electrostatic discharge, e.g., from a device or a human body, lightning, a build up of static electricity or be induced by the operation of other electronic or electrical components. An EOS transient can rise to its maximum amplitude in subnanosecond to microsecond times and have repeating amplitude peaks.
The peak amplitude of the electrostatic discharge (ESD) transient wave may exceed 25,000 volts with currents of more than 100 amperes. There exist several standards which define the waveform of the EOS transient. These include IEC 61000-4-2, ANSI guidelines on ESD (ANSI C63.16), DO-160, and FAA-20-136. There also exist military standards, such as MIL STD 883 part 3015.7.
Materials exist for the protection against EOS transients (“EOS materials”), which are designed to rapidly respond (i.e., ideally before the transient wave reaches its peak) to reduce the transmitted voltage to a much lower value and clamp the voltage at the lower value for the duration of the EOS transient. EOS materials are characterized by having high electrical impedance values at low or normal operating voltages and currents. In response to an EOS transient, the materials switch essentially instantaneously to a low electrical impedance state. When the EOS threat has been mitigated these materials return to their high impedance state. These materials are capable of repeated switching between the high and low impedance states, allowing circuit protection against multiple EOS events.
EOS materials also recover essentially instantaneously to their original high impedance value upon termination of the EOS transient. EOS materials can switch to the low impedance state thousands of times, withstanding thousands of ESD events, and recover to the high impedance state after providing protection from each of the individual ESD events.
Circuit components utilizing EOS materials can shunt a portion of the excessive voltage or current due to the EOS transient to ground, protecting the electrical circuit and its components. The major portion of a fast rise time transient, however, is reflected back towards the source of the threat. The reflected wave is either attenuated by the source, radiated away, or re-directed back to the surge protection device which responds to each return pulse until the threat energy is reduced to safe levels.
One electrical device for providing protection against EOS transients is disclosed in U.S. Pat. No. 6,211,554 B1, assigned to the assignee of this invention, and incorporated herein by reference. One voltage variable material (“VVM”) or composition for providing protection against electrical overstress is disclosed in U.S. patent application Ser. No. 09/136,507, assigned to the assignee of this invention, and is also incorporated herein by reference.
Typical local area network (“LAN”) design uses an Ethernet protocol, which usually requires a base band or a broad band transmission. Because LAN's typically encompass large distances between network devices (servers, work stations, printers, etc.), the ground potential may vary significantly from location to location. This might result in data transmission errors and even equipment damage if the data communication lines are referenced to earth ground. For this reason, transceivers for LAN and telecom applications are typically differential mode devices, usually isolated from the network wiring by transformers. These transformers efficiently couple the differential mode data signals from the twisted pair network wiring to the transceiver devices, while attenuating common mode signals such as those resulting form ground potential differences.
Another source of common mode signals are radiated transients from building power lines caused by load switching in equipment such as air conditioners, heaters, elevators, copiers and laser printers. It is also possible to couple common mode ESD signals to the network wiring by direct discharge to the cable or a cable connector, or by electric or magnetic field coupling to the cable.
Although an ideal transformer would couple a zero common mode signal from the primary (network wiring side) to the secondary (network device transceiver side), real transformers have some capacitance linking the primary and secondary windings, which allows some common mode current to flow across the transformer. Common mode chokes and differential mode transceivers further attenuate or reject the common mode signal, but high amplitude, fast rise signals such as those resulting from ESD may still cause system malfunction or damage.
Connection ports on network communication devices must be electrically isolated according to established standards. For example, Ethernet 10BaseT network communication devices must comply with International Standard ISO/IEC8802-3 (ANSI/IEEE Standard 802.3), and 100BaseT network communication devices must comply with the standards set forth in ANSI X3.263-1995, Section 8.4.1 1. For the 10BaseT devices, Standard 802.3 requires that for each PMA/MDI interface, such as that found in an RJ-45 connector, the connections to the network wiring must be isolated from ground for DC voltage levels as high as 2500 volts.
Known VVM type ESD apparatuses cannot withstand the presence of a high voltage, steady state signal. Other types of ESD protection, such as spark gaps, are less reliable. Spark gaps are subject to environmental conditions, such as heat and humidity. Moreover, spark gaps can degrade after repeated ESD events. This presents a problem for networked communication systems that require ESD protection and high voltage, direct current isolation. Accordingly, a need exists for a reliable VVM type ESD apparatus that can withstand the presence of a high voltage (e.g., 2500 VDC), steady state signal.